1. Field of the Invention
The present invention relates to an apparatus for electric induction heating of molten metal, which is designed to perform various types of metallurgical treatments including melting and heating of varied ferrous alloys and non-ferrous alloys.
2. Description of the Prior Arts
Conventional ladle-type furnaces for high-frequency or low-frequency coreless-type induction heating for metallurgical treatments of metals are structured such that an induction coil is arranged around the outer circumference of a refractory vessel in which molten metal is held. A yoke is provided outside the coil and the unit of these components is reinforced by an outer frame structure. The furnace and the coil are inseparably assembled so that each vessel is required to have its own coil incorporated therein. Therefore, at least two furnaces are required for successive operations; one for the actual operation and the other for repairation of the linings. This naturally leads to economical disadvantages.
In order to eliminate the economical disadvantages, it may be considered that the coil is evenly spaced in an arrangement around the outer shell of the furnace. For this purpose, however, it is necessary that the outer shell of the furnace is made of steel, or some other metal frame structure is used for the required strength.
This metal frame structure, or a steel or metal shell when arranged inside the coil, generates an induction current which causes a great heat loss hence markedly damaging the electrical efficiency in induction heating of the metal disposed at the center of the coil.
As a method for overcoming the above disadvantage, Japanese Laid-Open Patent Specification Sho No. 53-88603 discloses an arrangement of non-magnetic and non-oxidizable steel beams in the form of parallel pillars to function as a frame structure. This proposed structure is favorable in some respects, but in terms of its over-all efficiency, there is still room for improvement. Also, the structure is not a completely closed system, thus failing to function as a vacuum system.
When the induction heating is desired under a vacuum, it is theoretically prohibited to apply a voltage of 240 V or higher under a high degree of the vacuum according to Paschen's Law, etc. concerning the relation between the gas pressure and the atmosphere-insulated voltage to be applied.
Therefore, in order to apply a voltage of 240 V or higher in this case, there is no way but to increase the coil current, which is also limited due to the coil shape and other factors. In practical operations of induction heating, the problem has been that this vacuum discharge hinders the application of a high voltage required by the heating capacity necessary for practical operations. The application of excessive voltages would create the danger of breaking the insulation during charging or the danger of short-circuiting.
For the reasons mentioned above, it is strongly desired to perform the induction heating outside the vacuum atmosphere in which the charge is disposed, by arranging the coil outside the vacuum atmosphere and placing the furnace and charged metal under the vacuum atmosphere.
However, when the furnace is reinforced with a metal shell, etc. selected depending on the strength of the furnace required under the vacuum atmosphere, the magnetic flux is shielded by the metal shell so that the charge cannot be satisfactorily heated. Also when the furnace is made of non-magnetic materials, such as ordinary refractories, and heat-insulating materials, the desired air-tightness cannot be assured and the strength as required by a ladle cannot be maintained.